Roadway repair is often accomplished by overlaying the existing pavement (whether of concrete or asphalt paving material) with a new layer (often called a leveling course) of concrete or asphalt paving material. Without prior surface treatment, however, this method of repair generally results in the application of insufficient quantities of paving material in the rutted, potholed or otherwise damaged areas, because the overlay will be applied at the same rate per unit of roadway width in damaged areas (which have a greater depth to be filled across the width) as in the undamaged areas. The resulting reduced thickness in the overlay of the previously damaged areas will lead to renewed rutting or other wear damage in the new pavement in relatively short order. However, by milling the surface of the damaged pavement to a uniform surface elevation below the level of the damage, the addition of new pavement will produce a road surface having a consistent elevation across the entire width of the roadway. This repaving technique can be used to return the elevation of a damaged roadway to its original pre-damaged elevation, whereas the placement of a leveling course atop damaged but un-milled pavement will tend to raise the surface of the roadway or some portion thereof above its original elevation. Roadway repair without milling can require the raising of road shoulders, guardrails and manhole covers and the adjustment of overpass clearances, all of which is unnecessary if a proper milling technique is employed. A use of milling prior to repaving can also permit ready establishment of the proper road grade and slope, and thereby avoid drainage and safety problems. Furthermore, milling typically provides a rough surface that readily accepts and bonds with the new asphalt or other pavement overlay. Finally, milling can provide raw material that can be reclaimed for use in the production of new paving materials.
A milling machine typically comprises a wheeled or track-driven vehicle that includes a milling drum having a plurality of cutting teeth around its periphery, which milling drum is mounted for rotation about a substantially-horizontal axis within a drum housing on the frame of the machine. Steerable wheel-drive or track-drive assemblies operated by hydraulic motors are provided to drive the machine in a processing direction and to steer it along a desired milling path. The drive assemblies are attached to lifting columns that include internal linear actuators which can be activated to raise and lower the frame of the machine with respect to the roadway surface. Wheel-drive machines include four wheel-drive assemblies, one at the left front, one at the right front, one at the left rear and one at the right rear. Track-drive machines include three or four track-drive assemblies including one at the left front and one at the right front. Some such machines will also include a third track-drive assembly at the left rear and a fourth at the right rear; however, some track-drive machines will have only a single, center-mounted rear drive assembly.
Since the milling drum is mounted in a housing on the frame of the machine, raising the frame on the lifting columns can raise the milling drum out of contact with the roadway surface, and lowering the frame on the lifting columns can lower the milling drum into contact with the road surface so as to make a cut of the desired depth. The milling drum is rotated by a primary drum drive assembly typically comprising a drive belt driven by a diesel engine, which drive belt engages a sheave on an input drive shaft for the cutter drum. A gear box is typically located between the sheave and the milling drum and includes a gear train and an output drive shaft on which the milling drum is rotated. The gear box thus allows for rotation of the output drive shaft for the milling drum at a speed and torque that is different from that of the input drive shaft. Generally, the milling machine also includes a conveyor system that is designed to carry the milled material that has been cut from the roadway by the rotating milling drum to a location in front of, to the rear of, or beside the machine for deposit into a truck for removal from the milling site. Power for operation of the hydraulic motors that are typically employed to operate the conveyors and the drive assemblies is usually provided by the diesel engine.
A road stabilizer is similar to a milling machine in that it comprises a wheeled or track-driven vehicle that includes a rotating milling drum on which are mounted a plurality of cutting teeth, which drum is rotated by a primary drum drive assembly typically comprising a belt drive that engages a sheave on an input drive shaft for the cutter drum. A gear box is typically located between the sheave and the milling drum and includes a gear train and an output drive shaft on which the milling drum is rotated. The gear box thus allows for rotation of the output drive shaft for the milling drum at a speed and torque that is different from that of the input drive shaft. The wheel-drive or track-drive assemblies of the road stabilizer are mounted on lifting columns that include internal linear actuators which can be activated to raise and lower the frame of the machine with respect to the roadway surface. However, the milling drum of a road stabilizer is generally employed to mill or pulverized an existing road bed or roadway to a greater depth than does a milling machine prior to repaving (usually called reclaiming) or prior to initial paving (usually called stabilizing), and it leaves the pulverized material in place. The pulverized material left behind is usually compacted and covered with one or more additional layers of crushed aggregate material before paving.
Because the milling drums and the lifting columns of a milling machine and of a road stabilizer operate in the same way for purposes of this invention, the term “milling machine” will be used hereinafter as a generic term for both types of machines.
The milling drum of a milling machine is partially enclosed in a drum housing that prevents material being milled from the surface from being ejected away from the machine, at least partially controls the dust produced in the milling operation, and also protects against inadvertent access to the rotating milling drum. Conventional milling machines include end gates on opposite sides of this housing that float to the level of the roadway or other surface being milled when the milling drum is set to provide the desired cut depth. Thus, the end gates travel along the uncut surface of the roadway outside the path cut by the milling drum. However, conditions on the roadway may change as the milling machine progresses in milling the surface. Thus, the operator is constantly monitoring the positioning of the milling drum in order to maintain the desired cut.
It is known to use sensors mounted on the frame of a milling machine to allow an operator to control the extension of the front lifting columns; however, the rear lifting columns are manually adjusted by the machine operator without sensor input at the beginning of the milling process and when changes are required during the milling process. It would be desirable if an automatic system could be developed for automatically controlling the extension of the lifting columns throughout the milling process in order to control the elevation of the milling drum during a milling operation.